The present primary purpose in creating a high resolution pattern in a resist with light, e-beams or X-rays is to use the resulting image as a mask in the fabrication of devices, circuits, and other masks for pattern fabrications. The limit of resolution in such photofabrications depends on the physics of the system as well as on the fabrication techniques. For light photofabrication, diffraction blurring limits working resolution to about one micron. With ultraviolet light, the photon wavelength is slightly reduced and the resolution can be improved to about 0.75 micron. For e-beam lithography, the particle wavelength is very small, but the scattering broadening, particularly the electron back-scattering as from a silicon substrate, becomes a serious problem. In replication of masks, silicon can be replaced by lighter elements as substrates, and the e-beam approach can be most usefully applied.
For X-ray lithography, the particle wavelength is also very small. X-ray resists are electron sensitive, and X-rays are absorbed with the generation of various kinds of electrons which expose the resist in much the same way as an incident e-beam. Photon generated electrons (photoelectrons hereafter) as well as secondary X-rays contribute to the various broadening effects on the pattern and loss of resolution. A photoelectron can have an energy up to that of the incident X-rays and generally has a range of travel shorter than the X-rays for energies of more than a few keV. Photoelectrons tend to register singular latent images in a resist of small silver halide grains, while Auger electrons will create multiple latent images from the emulsion grain itself. In non-silver resists, photoelectrons have limited linear-energy-transfers (the energy deposited in a unit length of material through which the electron travels) while the Auger electrons, being very soft, give very high linear-energy-transfers and thus deposit all their energies in a very small region. Usually the path of travel of an Auger electron is less than 50 A.degree..
As many as 18 Auger electrons can be generated from a single Auger cascade of an Iodine atom, and as many as 13 Auger electrons from a Bromine atom. These Auger electrons create multiple latent images in a highly concentrated manner in contradistinction to photoelectrons whose latent images are far more disperse.
X-ray photons have much higher energy than light. Every ten fold increase in photon energy generally leads to a reduction of photon absorption cross-section by more than three orders of magnitude. Except for synchrotron radiation sources, conventional X-ray sources can hardly supply the resist with a brightness of more than a small fraction of a millijoule per cm.sup.2 -sec. Such a weak source requires a long exposure and provides very low throughput in commercial fabrications.
U.S. Pat. No. 4,350,755 teaches the use of relatively monochromatic X-ray of proper energy to induce inner shell ionization of a silver halide (excluding the flourides) resist, leading to a cascade of Auger electrons from the emulsion grain itself to form the desired latent image. The use of silver halide crystal provides a very large amplification factor as the photoreduction of only a few silver atoms of an emulsion grain can lead to the reduction of the entire silver halide crystal to elemental silver in the process of development. The fact that an over-exposed grain would reduce to about the same number of elemental silver atoms gives the silver halide resist a non-linear photosensitivity where the minimum exposure depends only on the grain size. The photoeconomy of this X-ray system is further improved by the selection or concentrating of the X-ray photons to be on an absorption edge of the halide in the resist. Multiple electrons from the grain itself can be induced from a single Auger cascade event. This improved photoefficiency greatly reduces the level of X-ray exposure required to imprint a latent image in the resist, and eliminates the radiation damage to photomasks which can be a very costly factor in a microfabrication processes.
In X-ray lithography, the beam usually consists of photons having a broad range of wavelengths. Secondary fluorescent photons as well as Compton scattered photons are created from the incident photons in the target material. These secondary photons can sensitize the resist with almost the same efficiency as the primary beam, producing an undesired broadening of the pattern image.
In Auger microlithography as here proposed the incident X-ray beam is relatively monochromatic with the photon energy being tuned to a certain photon absorption edge, for example the K or L-edge, of an inner electron shell of the sensitizing atomic element in the resist to be activated to produce the desired Auger electrons. The secondary photons, especially the Compton back-scattered photons, are largely of an energy level below the designated absorption edge and cannot trigger the Auger event. In the process of development, the sensitization by clustered Auger electrons can be distinguished from that of the scattered secondary photoelectrons. The images of the latter outside of the pattern areas can be treated like a background fogging, to be bleached back to a silver halide state for the silver halide resists, or remain undeveloped in a non-silver resist. By developing only the Auger images given by the primary beam, mask dimensions are faithfully reproduced in the resist.
X-rays have been used since their discovery to expose photosensitive materials (see "X-Ray Techniques and Registration Methods" by B. Fay, 1980, in Microcircuit Engineering, edited by Ahmed and Nixon). When irradiating a resist with X-rays, several factors concerning energies and their source need be considered. The preferred spectral region is between 0.37 to 60 A.degree., especially 2 and 50 A.degree.. Shorter wavelengths make the absorber, typically a gold mask, too transparent and require too large an aspect ratio (thickness to line-width) for fine lines, and longer wavelengths lose too much in absorption by the mask substrate. The spectral range can be extended with the use of heavier masking materials. Heavier elements such as uranium-238, for example, can reduce the thickness needed as compared to gold by one-third or more. An increase in the photon energy brings about, besides changes in the photocross-section, changes in the Auger yield, the Compton yield, and the shift of photon energy from large angle Compton scattering. X-rays can be obtained by synchrotron radiation, X-ray tubes, or radioactive line sources. Different sources produce different spectral intensities and brightnesses and require different capital expenditures, giving differing costs per photon.
For soft X-rays under one keV, the Auger yield (Auger against fluorescent) is high, the Compton yield (Compton scattering against photoabsorption) is low, and the Compton energy-shift at large-angle scatterings (the "Auger window") is small. "Auger window" as here used is a range of photoenergies for the incident X-rays to ionize an inner-shell electron of the sensitizing atomic element and produce the desired auger electrons, whereat the large-angle (80.degree. or larger) Compton scattered X-rays would have energies which fall below the inner-shell absorption-edge and cannot trigger the auger electrons. Requiring small Auger windows, soft X-rays can best be produced by synchrotron radiation where the spectral width can be maintained sufficiently narrow such that the large-angle Compton scattered photons do not have energies above the designated edge of the target atomic element in the resist and avoid the Auger event. With a sufficiently bright synchrotron source, the mask substrate can be thickened to improve its mechanical strength. For harder X-rays, the absorption cross-section is greatly reduced, the Auger yield is reduced, the Compton yield is increased, and the Auger window is increased. In other words, while the broadening by Compton scattered photons becomes important at higher X-rays energies, the Auger window also becomes large so that even with X-ray tubes, a chosen spectral line can be used to irradiate the resist and produce the latent Auger images only from the primary beam. Numerically, with a Compton scattering angle of .theta..perspectiveto.80.degree., the Compton wavelength shift, .delta..lambda.=0.024261(1-Cos .theta.), is 0.02 A.degree. and the energy shift, or the Auger window, is 1812 eV at 0.37 A.degree., 248 eV at 1 A.degree., 15.5 eV at 4 A.degree., 3.87 eV at 8 A.degree., and 0.155 eV at 40 A.degree..